This invention relates to apparatuses and methods for checking specific signal inputs usable, for example, for telephones, more particularly apparatuses and methods for checking inputs of specific signals transmitted as audio signals.
In general telephone communication systems, each telephone transmits/receives talking voices via a public line. In recent years, however, various additional services are developed and put in practical use for such communications.
One of those services is, for example, a service sending caller's data such as telephone number, name, etc. (hereafter, to be referred to as caller ID) to the listener's telephone and enabling the listener to check the caller ID on his/her telephone when receiving the call.
For example, when a listener's telephone is ringing, the telephone station sends the caller ID to the telephone. The telephone decodes the caller ID and displays the telephone number, name, etc. included in the caller ID on the display unit. Then, the listener can check the caller before he/she answers the phone. This will be very helpful for the listener to judge whether to answer the phone and to prevent mischief calls.
In this caller ID sending service, the subject telephone station sends a CAS (Call Alert Signal) to the listener's telephone, notifying that the caller ID is to be sent at first.
Receiving the CAS, the listener's telephone outputs a signal to the telephone station notifying that the telephone is now ready to receive the caller ID. Receiving the signal, the station begins sending of the actual caller ID to the telephone.
Not only at the beginning of the calling, but such a caller ID may be also sent during talking sometimes.
In other words, the service is combined with a call waiting service.
FIG. 1 shows a flow chart of such a service operation.
Assume now that a telephone A is connected to another telephone B for talking through a public line via a telephone station as shown as (S1) in FIG. 1.
If another telephone C makes a call to the telephone A in such a case as shown as (S2) in FIG. 1, then the telephone station sends a ringing sound to the telephone A, as a call waiting service, so that the user of the telephone A can know that another telephone is making a call to his/her telephone A even while he/she is talking with the telephone B. In this case, the user of the telephone A can know who the present caller is by the caller ID of the telephone C sent by the telephone station.
In order to send this caller ID, the telephone station mutes the caller voice (S3) sent from the telephone B to the telephone A and sends a CAS to the telephone A (S4).
Detecting the CAS (S5), the telephone A mutes the voice being sent to the telephone B (S6) to which the user of the telephone A is talking, then sends a CAS confirmation signal to the telephone station (S7).
Receiving the CAS confirmation signal, the telephone station sends the caller ID of the telephone C (S8) to the telephone A, then resets the mute (S9). On the other hand, the telephone A resets the mute (S10) when receiving the caller ID, with which the communication between telephones A and B is restored to the normal status (S11).
It is only 0.3 to 0.4 sec that the communication is muted and disconnected temporarily in such an operation. Thus, this will not affect the communication between telephones A and B actually at all.
Receiving the caller ID, the telephone A decodes the caller ID and displays the number, user name, etc. of the telephone C on the display unit (S12). Consequently, the user of the telephone A can know the caller when a call waiting service is activated during talking.
It is well known that general public lines can transmit audio range signals. To transmit such signals as a dial tone, etc. those public lines use dual tone multifrequency signals (DTMF signals). In other words, one of low frequencies and one of high frequencies are selected respectively for each of numerals and characters and they are combined into a tone signal (dual tone) and transmitted.
The above mentioned CAS and CAS confirmation signal are also defined as dual tone signals. In other words, they are defined as specific signals each of which is generated by a combination of two frequencies. For example, the CAS is defined as a signal comprising two signal components f1 and f2 (frequencies) as shown in FIG. 2A.
So-called caller ID data is sent as an FSK signal.
Even when receiving a CAS from a telephone station during talking as mentioned above, the telephone A is expected to detect the CAS correctly. On the other hand, the telephone A cannot detect any signal other than a CAS as a CAS.
FIG. 3 shows a CAS detecting circuit configuration provided in a telephone so as to detect a CAS comprising signal components as shown in FIG. 2A.
Terminals L.sub.1 and L.sub.2 are connected to a public line. Signals sent via a public line are entered to the balanced input amplifier 50 provided in the CAS detector shown in FIG. 3 via DC blocking capacitors C1 and C2, as well as via input resistors R1 and R2. The resistor R1 and the capacitor C1, as well as the resistor R2 and the capacitor C2 are combined to form a high-pass filter respectively.
The output from the balanced input amplifier 50 is supplied to the band-pass filters 52 and 53 after its range is limited in the low-pass filter 51. The band-pass filter 52 has a narrow range pass band whose center frequency is f1 shown in FIG. 2A. The band-pass filter 53 has a narrow range pass band whose center frequency is f2.
Consequently, if a CAS as shown in FIG. 2A is sent from a public line, for example, then both the band-pass filters 52 and 53 output signals over a certain level. The detection judging circuit 54 monitors the output from both the band-pass filters 52 and 53. The circuit 54 can obtain both outputs over a certain level, that is, detects that a signal containing both f1 and f2 is entered to the CAS detector. The circuit 54 then outputs the detected information to the time constant circuit 55. The time constant circuit 55 judges a CAS input when both signal components of frequencies f1 and f2 are recognized for over a certain time, then outputs a CAS detection signal S.sub.DET.
This CAS detection signal S.sub.DET is supplied to the controller comprising a microcomputer, etc. in the telephone. The controller controls operations as required according to the CAS detection signal S.sub.DET. In other words, the controller controls the operations in and after (S5) in FIG. 1.
On the other hand, as for a CAS detection during talking, the circuit as shown in FIG. 3 may make a wrong detection of CAS.
In other words, although a CAS comprises two frequencies in the audio signal range, it is sent as if it were talking voices on the public line during talking. And there are also other talking voice signals containing those two frequencies f1 and f2.
Cider a white noise as shown in FIG. 4A now as a signal containing signal components of frequencies f1 and f2 on a certain level.
If such a white noise is entered to the CAS detector shown in FIG. 3, it becomes, for example, a signal having a range as shown in FIG. 4B when it passes the low-pass filter 51. Furthermore, when passing the band-pass filters 52 and 53, it becomes a signal containing frequencies as shown in FIG. 4C. In other words, it becomes a signal having the same range components as those of the CAS shown in FIG. 2A. In this case, the white noise is mistaken as a CAS.
There is also a case in which audio signals containing frequencies f1 and f2 are transmitted on a public line as normal talking voices (voices output from this telephone or entered to the telephone). As understand from the case shown in FIG. 4, however, when such voices are sent at top volume, signals of frequencies f1 and f2 are extracted from those voice signals and detected wrong as a CAS input sometimes.